1. Field of the Invention
The present invention, relates to a certain crosslinked rubber particle and a rubber composition comprising as essential components such a crosslinked rubber particle and a non-crosslinked rubber component. A rubber composition of the invention exhibits an excellent processability, and also exhibits excellent handling performance and dimension stability, and the like especially when combined with a silica as a reinforcing agent. A vulcanized rubber obtained by vulcanizing a rubber composition of the invention can be used in various industrial field as a rubber product such as a belt, a hose, a roll, a tire, and the like.
2. Description of the Related Art
A starting rubber which has been employed frequently for a rubber product may for example be:    (A) a polymer whose repeating unit consists only of a conjugated diene unit such as a butadiene rubber, an isoprene rubber, a chloroprene rubber and a natural rubber;    (B) a polymer whose repeating units are a conjugated diene unit and an aromatic vinyl unit such as a styrene-butadiene copolymeric rubber, a styrene-isoprene copolymeric rubber and a styrene-isoprene-butadiene copolymeric rubber;    (C) a polymer whose repeating units are a conjugated diene unit and an α,β-unsaturated nitrile unit such as an acrylonitrile-butadiene copolymeric rubber;    (D) a polymer in which an acrylate unit is essential as a repeating unit such as an acrylic rubber; and,    (E) a polymer whose repeating units are ethylene and an α-olefin having 3 to 12 carbon atoms optionally with a non-conjugated polyene, such as an ethylene-propylene rubber.
Any of these starting rubbers, when combined with a silica as a reinforcing agent, can provide a colored rubber product except for a black colored product, and a favorable performance of a formulated rubber made therefrom is a heat generation kept at a low level. However, unlike to a carbon black, a silica may lead to a problematic reduction in the tensile strength and the wearing resistance of a vulcanized rubber, and the like. Additional problems associated with the incorporation of a silica include {circle around (1)} a reduction in the processability and the compound consistency upon kneading, {circle around (2)} an insufficient smoothness of the surface of a sheet made from a compound using a roll and an irregular sheet edge accompanied with a substantial shrinkage upon cooling, and the like.
Such problems may be due to an aggregation of a silica instead of a sufficient dispersion. A rubber composition in which a silica is not dispersed sufficiently poses a difficulty in obtaining a certain reinforcing effect of the incorporation of a reinforcing agent and undergoes a heat generation attributable to the interaction between poorly dispersed silica particles, resulting in a substantial reduction in the processability. Such poor dispersion of a silica occurs possibly because of the reasons that a silica has a high self-interacting effect when compared with a carbon black and tends to aggregate easily and that a starting rubber usually consists of carbon and hydrogen and has a lower polarity when compared with a silica which leads to a low affinity with the silica.
For the purpose of increasing the affinity of a silica with a hydrocarbon-based rubber, the use of a conjugated diene-based rubber to which a functional group having an affinity with the silica is introduced has been investigated. For example, a conjugated diene-based rubber to which a hydroxyl group is introduced (WO96/23027), a conjugated diene-based rubber to which an alkoxysilyl group is introduced (JP-A 9-208623) and a conjugated diene-based rubber to which an alkoxysilyl group and an amino group or a hydroxyl group are introduced (JP-A 9-208633) have been proposed. However, such conjugated diene-based rubber to which a functional group described above is introduced mostly exhibits problematically poor dispersion, excessive heat generation upon processing and poor processability, and the like since it undergoes an extensive aggregation with a silica when the silica is admixed.
On the other hand, a silane coupling agent is incorporated usually for the purpose of preventing an aggregation of a silica and achieving a uniform dispersion. This results in an improvement in the dispersibility of a silica to some extent and a substantial improvement in the processability of a rubber composition and the tensile strength and the wearing resistance of a vulcanized rubber and the like. Nevertheless, the smoothness of the surface and the edge of a sheet or the problematic shrinkage of a sheet upon cooling or the like has not adequately been improved. Furthermore, a sufficient improvement in the processability, the tensile strength or the wearing strength or the like requires the incorporation of a silane coupling agent in an amount as high as 8 to 15% by weight based on a silica, but such silane coupling agent is. expensive and increases the cost for a rubber composition.